The present invention relates to improvements in the performance of carbon-supported catalysts in solid polymer fuel cells. In particular, the invention relates to surface treatments for improving the performance of carbon-supported catalysts, especially at the cathode.
Fuel cell systems are potentially more efficient and cleaner than present day power supplies that burn fossil fuels. As a result, much effort has recently been directed towards developing fuel cell systems that are suitable for consumer use over a wide range of applications, from the small (for example, portable 1 kilowatt size generators) to the large (for example, automotive engines or stationary power plants). One of the development objectives relates to lowering costs so that fuel cell systems can be competitive with traditional fossil fuel burning alternatives.
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black, an alloy or a supported metal-based catalyst, for example, platinum on carbon particles.
A particularly attractive fuel cell is the solid polymer electrolyte fuel cell that employs a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Solid polymer fuel cells operate at relatively low temperatures (circa 80xc2x0 C.) compared to other fuel cell types.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical oxidation that occurs at the anode electrocatalyst of a solid polymer electrochemical fuel cell results in the generation of cationic species, typically protons and electrons. For an electrochemical fuel cell to utilize the ionic reaction products, the ions must be conducted from the reaction sites at which they are generated to the electrolyte. Accordingly, the electrocatalyst is typically located at the interface between each electrode and the adjacent electrolyte.
Effective electrocatalyst sites are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. For example, if the fuel stream supplied to the anode is hydrogen, electrons and protons are generated at the anode electrocatalyst. The electrically conductive anode is connected to an external electric circuit that conducts an electric current from the anode to the cathode. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode. Electrocatalyst sites that are ionically isolated from the electrolyte are not productively utilized if the protons do not have a mechanism for being ionically transported to the electrolyte. Accordingly, coating the exterior surfaces of the electrocatalyst particles with ionically conductive coatings has been used to increase the utilization of electrocatalyst exterior surface area and increase fuel cell performance by providing improved ion conducting paths between the electrocatalyst surface sites and the electrolyte.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Increasing effective utilization of the electrocatalyst surface area enables the same amount of electrocatalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance.
Although only small amounts of catalyst are used at the electrode/electrolyte interfaces, the usual catalyst materials are expensive and they can represent a substantial fraction of the overall fuel cell cost. For instance, solid polymer fuel cells commonly employ a platinum catalyst at the cathode and a platinum-ruthenium catalyst alloy at the anode. It is therefore important to use catalyst material as efficiently as possible. This includes increasing utilization, (for example by increasing the available active catalyst surface per unit weight of catalyst).
U.S. Pat. No. 5,084,144 discloses a method for making fuel cell electrodes. The catalyst is electrochemically deposited on the surface of the electrodes via a layer of proton-conducting polymer electrolyte. As a result, a thin layer of catalyst is only deposited where it is accessible to a proton-conducting electrolyte. Such an electrochemical deposition technique may be difficult, however, to implement economically in large scale production.
More commonly, catalyst is incorporated in fuel cell electrodes by applying a high surface area catalyst powder in a thin layer on an electrode substrate. The catalyst powder is typically mixed with an ionomer electrolyte solution to form an ink that is applied to the electrode substrate. The ionomer in the ink remains in intimate contact with the catalyst after removal of the solvent and thus provides electrolyte access to the dispersed catalyst.
A preferred way to obtain a high surface area catalyst powder is to disperse the desired catalyst on the surface of a larger particulate support. High surface area carbon blacks are preferred supports in this regard. It is relatively economical to prepare electrodes comprising a thin layer of carbon-supported catalyst coated on an electrode substrate. While the utilization of the catalyst in a typical fuel cell electrode is satisfactory, it is nevertheless desirable to improve the utilization.
Carbon-supported catalysts have historically been prepared by depositing the metal-based catalyst onto the porous carbon support material surface. Recently, improved ion exchange methods have been adopted to prepare certain carbon-supported catalysts. See, for example, Ann. Chim. Sci. Mat., 1998, Vol. 23, p331-335. In these methods, a suitable carbon support is treated with a strong oxidizing solution thereby creating many active acidic surface oxide groups on the carbon surface which can serve as ion exchange sites. Surface oxides which can form on carbon and specifically acidic surface oxides which can form on carbon are discussed in Table 3.1, pages 86-88 in xe2x80x9cCarbon, electrochemical and physicochemical propertiesxe2x80x9d, K. Kinoshita, John Wiley and Sons, Inc., New York, U.S.A., 1988. Acidic surface oxide groups include carbonyl, carboxylic, phenolic, quinones, lactones, groups containing one or two oxygen atoms, and the like. A slurry is made which contains the treated carbon support and a solution containing a suitable metal catalyst salt or complex. Cations from the solution (e.g. Pt(NH4)32+) exchange with protons at the active sites. Afterwards, the sample is heated in air to form platinum oxides and is then heated in a reducing environment (for example, hydrogen) to convert the bound platinum species to an active metallic form. This method desirably results in highly dispersed, small particle platinum deposits exhibiting a high surface area. The reducing treatment to form platinum metal, however, removes most of the remaining surface oxide groups on the carbon support.
In solid polymer fuel cell applications, significant improvement can be achieved in the performance of electrodes incorporating carbon-supported catalysts. It has been discovered that electrode, and therefore fuel cell performance, is enhanced when the surface of the carbon-supported catalyst comprises more acidic surface oxide groups than in conventional carbon-supported catalysts. Preferably, there should be sufficient acidic surface groups such that the pH of the carbon-supported catalyst is less than about 5. A particularly significant performance improvement may be obtained when the pH of the carbon-supported catalyst is less than about 3. In particular, the performance of a fuel cell cathode supplied with a gaseous reactant can be enhanced. Preferably the carbon-supported catalyst comprises platinum and the carbon support is an acetylene or furnace black.
Introducing more than the conventional amount of acidic surface oxide groups on the carbon-supported catalyst can be accomplished by chemically treating the carbon-supported catalyst with an oxidant, such as, for example, a solution comprising an oxidizing species. The oxidizing chemical treatment is performed on the carbon-supported catalyst after the metal-based catalyst has been deposited on the carbon.
In principle, any preferred oxidizing species may be employed in the treatment solution, including HNO3, H3PO4, KMnO4, KClO3, HF, or (NH4)2S2O8. Preferably a strong oxidizing solution is used. Preferred acidic solutions include HNO3 in concentrations greater than about 4M or H3PO4 in concentrations greater than about 5M. Successful results can be obtained with the former by treating at room temperature or above for more than about 1 hour, or with the latter by treating above about 100xc2x0 C. for more than about 1 hour. Lower treatment temperatures are preferred in order to prevent Ostwald ripening. (Ostwald ripening refers to the tendency of smaller catalyst particles to coalesce into larger particles, thereby reducing the active area.)
After treating with an oxidizing solution, the residual solution may be removed simply by filtration and washing the oxidized carbon-supported catalyst in water, drying the oxidized carbon-supported catalyst filter cake, and then grinding the cake to obtain free flowing oxidized carbon-supported catalyst. The drying can be performed below about 80xc2x0 C. in air.
While low temperature chemical treatments are preferred, other techniques may be employed to reduce the pH of (that is, by introducing more than the conventional amount of acidic surface groups on) a carbon-supported catalyst. For example, this may be achieved by omitting or modifying steps in the conventional preparation of carbon-supported catalysts. For instance, sufficient acidic surface groups may exist on the carbon support during ion exchange deposition of the metal-based catalyst but, as described above, they are typically subsequently removed by post-deposition processing steps. Omitting or modifying these post-processing steps may also produce suitable catalysts with lower than conventional pH.
In fuel cell electrodes, a carbon-supported catalyst is typically applied to an electrode substrate (typically a porous, electrically conductive sheet material) and/or to the membrane. Often, carbon-supported catalysts are applied in the form of an ionomer ink, which comprises an ionomer solution (e.g. an aqueous solution of a poly (perfluorosulphonic acid), and the oxidized carbon-supported catalyst. In this way, the ionomer is in intimate contact with the carbon-supported catalyst. The oxidative treatment described herein is preferably performed before the carbon-supported catalyst is made into an ionomer ink and applied to the electrode substrate, or otherwise incorporated in a fuel cell electrode.